SCR catalysts having improved low temperature performance, and methods of making and using the same

ABSTRACT

SCR-active molecular-sieve based catalysts with improved low-temperature performance are made by heating a molecular-sieve in a non-oxidizing atmosphere with steam (hydrothermal treatment), or in a reducing atmosphere without steam (thermal treatment), at a temperature in the range of 600-900° C. for a time period from 5 minutes to two hours. The resulting SCR-active iron-containing molecular sieves exhibit a selective catalytic reduction of nitrogen oxides with NH 3  or urea at 250° C. that is at least 50% greater than if the iron-containing molecular-sieve were calcined at 500° C. for two hours without performing the hydrothermal or thermal treatment.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates generally to molecular sieve based-catalysts usedin selectively converting nitrogen oxides (NO_(x)) present in a gasstream to nitrogen using a nitrogenous reductant such as ammonia (NH₃)and in particular it relates to Fe-containing catalysts which areparticularly active at relatively low temperatures in relation toconventional Fe zeolite catalysts. The molecular sieve in thesecatalysts is preferably a zeolite or a silicoaluminophosphate (SAPO).

Description of Related Art

Selective catalytic reduction (SCR) systems utilize NH₃ as a reductantto reduce NO_(x) to elemental nitrogen. A principal application of SCRtechnology is in the treatment of NO_(x) emissions from internalcombustion engines of motor vehicles, and especially lean-burn internalcombustion engines. SCR systems are also applied to static sources ofNO_(x), such as power plants.

One class of SCR catalysts is transition metal exchanged zeolites.Vanadium-based SCR catalysts are unsuited for higher temperatureenvironments due to their thermal instability. This has led to thedevelopments of copper and iron promoted zeolites. Copper zeolitecatalysts achieve high NO_(x) conversion (90% or more) at relatively lowtemperatures (from about 180 to about 250° C.), but they require theinjection of greater amounts of urea to be effective at relativelyhigher temperatures (greater than about 450° C.). Conventional ironzeolite catalysts achieve high conversion (90% or more) of NO_(x) attemperatures over 350° C., but at lower temperatures more typical forexample of normal diesel engine exhaust (about 180 to about 250° C.),high conversions (up to about 90%) are obtained only in the presence ofhigh levels of NO₂ (50% of the total NOx levels, i.e. 1:1 NO₂:NO).

It would therefore be desirable to provide SCR catalysts having improvedlow temperature (from about 200 to about 300° C.) performance.

SUMMARY OF THE INVENTION

The present invention reflects the inventors' surprising discovery thattreating an iron-containing molecular sieve at a relatively hightemperature (about 600° C. to about 900° C.) for a period of time fromabout 5 minutes to about two hours, in a non-oxidizing atmosphere and inthe presence of steam (hydrothermal treatment), improves the dispersionof the iron to the ion-exchange sites of the molecular sieve, andthereby improves the low-temperature (from about 150° C. to about 300°C.) performance of the iron-containing molecular sieve. This effect isparticularly advantageous in connection with zeolites or SAPOs of mediumand smaller pore size, such as ferrierite, in which the dispersion ofiron to the ion-exchange sites is otherwise hindered to a greater extentthan for larger pore zeolites.

The inventors have also discovered that treating an iron-containingmolecular sieve, preferably a zeolite or a SAPO, at a relatively hightemperature (about 600° C. to about 900° C.) for a period of time fromabout 5 minutes to about two hours, in a reducing atmosphere without thepresence of steam (thermal treatment) also improves the dispersion ofthe iron to the ion-exchange sites of molecular sieve, preferably azeolite or a SAPO, and thereby improves the low-temperature (from about150° C. to about 300° C.) performance of the iron-containing molecularsieve.

Thus, in one aspect, the invention relates to a method of making anSCR-active molecular sieve based-catalyst, preferably a zeolite or aSAPO, comprising performing a hydrothermal treatment on aniron-containing molecular sieve, preferably a zeolite or a SAPO, in anon-oxidizing atmosphere at a temperature in the range of about 600° C.to about 900° C. for a time period from about 5 minutes to about twohours.

In another aspect, the invention relates to a method of making anSCR-active molecular sieve, preferably a zeolite or a SAPO, comprisingperforming a thermal treatment on an iron-containing molecular sieve,preferably a zeolite or a SAPO, in a reducing atmosphere at atemperature in the range of about 600° C. to about 900° C. for a timeperiod from about 5 minutes to about two hours.

As used herein, the term “hydrothermal treatment” means heating thematerial to high temperatures (about 600° C. to about 900° C.) in anon-oxidizing, i.e. inert or reducing, atmosphere, in the presence ofsteam.

As used herein, the term “thermal treatment” means heating the materialto high temperatures (about 600° C. to about 900° C.) in a reducingatmosphere without the presence of steam.

As used herein, the term “calcine”, or “calcination”, means heating thematerial in air or oxygen. This definition is consistent with the IUPACdefinition of calcination. (IUPAC. Compendium of Chemical Terminology,2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson.Blackwell Scientific Publications, Oxford (1997). XML on-line correctedversion: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat,B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.doi:10.1351/goldbook.) Calcination is performed to decompose the metalsalt and promote the exchange of metal ions with the molecular sieve andalso to adhere the catalyst to a substrate. The temperatures used incalcination depend upon the components in the material to be calcinedand generally are between about 400° C. to about 900° C. In applicationsinvolving the molecular sieves in the processes described herein,calcinations are generally performed at temperatures from about 450° C.to about 500° C.

Unless specified otherwise, the term “low temperature”, when used indescribing the performance of the catalyst, means a temperature fromabout 150° C. to about 300° C.

Unless specified otherwise, the term “high temperature”, when used indescribing the performance of the catalyst, means about 450° C. to about600° C. The term “high temperature”, when used in describing thehydrothermal treatment of the catalyst, means about 600° C. to about900° C.

As used herein, the term “about” means approximately. Approximatinglanguage, as used throughout the specification and claims, may beapplied to modify any quantitative representation that could permissiblyvary without resulting in a change in the basic function to which it isrelated. Accordingly, a value modified by a term such as “about” is notto be limited to the precise value specified. With regard to the use ofthe term “about” and specific numerical values encompassed by the term,the number of significant figures, the precision of the value and thecontext in which the term is used are important in determining thenumerical values associated with the term. For example, if a series ofmeasurements are taken over a temperature range from 300° C. to 500° C.,where the measurements are made at 25° C. intervals, the term “about400° C.” would encompass the range from 387° C. to 412° C., inclusive.When “about” is used in describing units of time in hours, the statedvalue includes a range of plus or minus 15 minutes, inclusive. Forexample, “about 2 hours” is meant to include time from 1 hour 45 minutesto 2 hours 15 minutes, inclusive. When “about” is used in describingunits of time in minutes, the stated value includes a range of plus orminus 8 minutes, inclusive. For example, “about 30 minutes” is meant toinclude time from 22 minutes to 38 minutes, inclusive.

DETAILED DESCRIPTION OF THE INVENTION

It has been found that hydrothermal or thermal activation of molecularsieves, especially small pore/medium pore iron-containing molecularsieves, especially zeolites and silicoaluminophosphates (SAPOs) resultsin a material suitable for iron-based selective catalytic reduction thatcan achieve greater than 90% NOx conversion efficiency at 200° C. in 75%NO₂/NOx with fast transient response to NH₃ fill. A transient responseis defined as the rate of increase in NOx conversion as the catalystfills with NH₃ from zero ammonia exposure to saturated ammonia exposure.

Without wishing to be held to any particular theory, the effect isattributed to the redispersion of iron to the exchange sites andcreation of new more active sites via Fe-molecular sieve, preferablyFe-zeolite or Fe-SAPO interactions induced by the high temperaturetreatment.

The improved iron-containing molecular sieves, preferably a zeolite or aSAPO, described herein thus provide increased NOx conversion without therequirement for large amounts of NH₃ prefill in high NO₂/NOx (75%)environments. In contrast, iron zeolites prepared by calcining for fivehours at 750° C. in air require large amounts of NH₃ to be dosed andstored on the catalyst to achieve a fill level of 0.8 g/L to reachacceptable conversion levels at 200° C.

Non-limiting examples of the types of exhaust gases that may be treatedwith the disclosed iron-containing molecular sieves include automotiveexhaust, including from diesel engines. The disclosed iron-containingmolecular sieves are also suitable for treating exhaust from stationarysources, such as power plants, stationary diesel engines, and coal-firedplants.

The iron-containing molecular sieves, preferably a zeolite or a SAPO, ofthe invention may be provided in the form of a fine powder which isadmixed with or coated by a suitable refractory binder, such as alumina,bentonite, silica, or silica-alumina, and formed into a slurry which isdeposited upon a suitable refractory substrate. The carrier substratemay have a “honeycomb” structure. Such carriers are well known in theart as having a many fine, parallel gas flow passages extendingtherethrough.

The iron-containing molecular sieve, preferably a zeolite or a SAPO, mayundergo hydrothermal or thermal treatment in either powder form or as anadmixture with other components in a form such as a coating, anextrudate, etc.

Hydrothermal or thermal treatment is performed on an iron-containingmolecular sieve, preferably a zeolite or a SAPO, at a temperature in therange of about 600° C. to about 900° C. for a time period from about 5minutes to about two hours.

When the zeolite is a ZSM-5, the hydrothermal or thermal treatment isperformed at a temperature in the range of from about 600° C. to about800° C., more preferably from about 670° C. to about 730° C., even morepreferably from about 670° C. to about 730° C., and yet even morepreferably about 700° C., for a time period from about 5 minutes toabout two hours.

When the zeolite is ferrierite, the hydrothermal or thermal treatment isperformed at a temperature in the range of from about 700° C. to about900° C., more preferably from about 750° C. to about 850° C., even morepreferably from about 770° C. to about 830° C., and yet even morepreferably about 800° C., for a time period from about 5 minutes toabout 2 hours.

Preferably, small pore and medium pore molecular sieves, includingzeolites and SAPOs can be used. One of ordinary skill in the art wouldbe able to determine the combinations of temperatures and times thatthese molecular sieves would need to undergo hydrothermal or thermaltreatment for commercial and/or optimal performance.

The non-oxidizing atmosphere is an inert or reducing atmosphere.Preferably the inert atmosphere comprises nitrogen. Preferably thereducing atmosphere comprises hydrogen. When the treatment ishydrothermal, steam is also present. When the treatment is thermal,steam is not present.

The method can further comprise introducing oxygen into thenon-oxidizing atmosphere after the hydrothermal or thermal treatmentduring a stage where the temperature decreases from the temperature ofthe hydrothermal treatment to ambient temperature (about 25° C.).

The hydrothermal or thermal treatment can be performed in a rotarycalcination oven in which an iron-containing molecular sieve, preferablya zeolite or a SAPO, is exposed, preferably in a powder form, to acountercurrent flow of gas constituting the non-oxidizing or reducingatmosphere.

The iron-containing molecular sieve, preferably a zeolite or a SAPO,subjected to the hydrothermal or thermal treatment can be in the form ofan extruded unsupported catalyst or a coating on an inert substrate.

The iron-containing molecular sieve, preferably a zeolite or a SAPO, isprepared by mixing a molecular sieve with an iron salt. The mixing cancomprise impregnating a molecular sieve, preferably a zeolite or a SAPO,with a solution of an iron salt; liquid phase ion-exchange of a slurryof a molecular sieve, preferably a zeolite or a SAPO, with a solution ofan iron salt spray-drying a slurry of a molecular sieve, preferably azeolite or a SAPO, and a solution of an iron salt; or by combining amolecular sieve, preferably a zeolite or a SAPO, and an iron salt viasolid-state mixing techniques. Impregnating a molecular sieve with asolution of an iron salt can be performed using such techniques asincipient wetness impregnation and wet impregnation. These solid-statetechniques range from simple loose mixing and grinding through to highenergy mixing methods, such as ball milling.

The molecular sieve, preferably a zeolite or a SAPO, can be pre-treatedin an oxidizing atmosphere at a temperature in the range of about 500°C. to about 800° C. for a time period from about one hour to about threehours, prior to impregnating the molecular sieve, preferably a zeoliteor a SAPO, with a solution of an iron salt and then receivinghydrothermal or thermal treatment.

The iron-containing molecular sieve is preferably small or medium pore.Preferred small or medium pore molecular sieves include zeolites andSAPOs. Preferred molecular sieves include BEA (beta-zeolite), MFI(ZSM-5), FER (ferrierite), CHA (chabasite), AFX, AEI, SFW, SAPO-34,SAPO-56, SAPO-18 or SAV SAPO STA-7.

In another aspect, the invention also relates to a process of making acatalyst module for abating nitrogen oxides in a gas stream by selectivecatalytic reduction. A catalyst module is a device containing a catalystwithin a housing where the housing comprises one or more inlets for thegas stream to enter the housing, and one or more outlets for the gas toexit after passing through the catalyst in the housing. The process ofmaking the catalyst module comprises combining a molecular sieve,preferably a zeolite or a SAPO, with at least one ionic iron species andat least one organic compound to form a mixture, calcining the mixtureand removing the at least one organic compound, forming a catalyststructure by extruding the calcined mixture into a substrate or coatingthe calcined mixture onto a substrate and mounting the catalyststructure within a housing having one or more inlets for gas to betreated with a reductant such as ammonia or urea in selective catalyticreduction. A catalyst module can also be made by a process comprisingpreparing a washcoat by forming a mixture comprising a molecular sieve,preferably a zeolite or a SAPO, at least one ionic iron species and atleast one organic compound, applying the washcoat to a substrate,calcining the coated mixture and removing the at least one organiccompound to form a catalytic structure, and mounting the catalyticstructure within a housing having one or more inlets for gas to betreated with a reductant such as ammonia or urea in selective catalyticreduction.

In another aspect, the invention relates to a method of increasingNO_(x) conversion in an exhaust gas by contacting an exhaust gascontaining NO_(x) with an iron-containing molecular sieve that had beentreated at a temperature from about 600° C. to about 900° C. for aperiod of time from about 5 minutes to about two hours, in anon-oxidizing atmosphere and in the presence of steam (hydrothermaltreatment) or in a reducing atmosphere without the presence of steam(thermal treatment). The method can increase NO_(x) conversion over thetemperature range of 175-300° C. that is at least twice the conversionobtained using a comparable catalyst that had not undergone hydrothermalor thermal treatment but had been calcined at 500° C. for two hours. Themethod can also increase NO_(x) conversion over the temperature range of175-250° C. that is at least about three times the conversion obtainedusing a comparable catalyst that had not undergone hydrothermal orthermal treatment but had been calcined at 500° C. for two hours. Themethod can also increase NO_(x) conversion over the temperature range of250-300° C. that is at least about two times, preferably at least aboutthree times the conversion obtained using a comparable catalyst that hadnot undergone hydrothermal or thermal treatment but had been calcined at500° C. for two hours. The method can also increase NO_(x) conversionover the temperature range of 200-250° C. that is at least about twotimes, preferably at least about three times the conversion obtainedusing a comparable catalyst that had not undergone hydrothermal orthermal treatment but had been calcined at 500° C. for two hours.

The invention also relates to a method of increasing NO_(x) conversionin an exhaust gas by contacting an exhaust gas containing NO_(x) with aniron-containing molecular sieve that had been treated at a temperaturefrom about 600° C. to about 900° C. for a period of time from about 5minutes to about two hours, in a non-oxidizing atmosphere and in thepresence of steam (hydrothermal treatment) or in a reducing atmospherewithout the presence of steam (thermal treatment), where thetemperatures needed for 10, 50 and 90% NOx conversion in catalystssubjected to hydrothermal treatment are about 170, 240 and 280° C.,which is at least 40° C. lower than the temperature needed to the sameconversion using a comparable catalyst that had not undergonehydrothermal or thermal treatment (about 220, 300 and 350° C.). Themethod also provides for maximum NO_(x) conversion at about 310° C.,which is at least 60° C. lower than the temperature needed to maximumconversion in a comparable catalyst that had not undergone hydrothermalor thermal treatment (about 375° C.).

SCR-active iron-containing molecular sieves, preferably a zeolite or aSAPO, that had been treated at a temperature from about 600° C. to about900° C. for a period of time from about 5 minutes to about two hours, ina non-oxidizing atmosphere and in the presence of steam (hydrothermaltreatment) or in a reducing atmosphere without the presence of steam(thermal treatment), can increase NO_(x) conversion over the temperaturerange of 175-300° C. that is at least twice the conversion obtainedusing a comparable catalyst that had not undergone hydrothermal orthermal treatment but had been calcined at 500° C. for two hours. TheSCR-active iron-containing molecular sieves, preferably a zeolite or aSAPO, that had received hydrothermal treatment or thermal treatment, canalso increase NO_(x) conversion over the temperature range of 175-250°C. that is at least about three times the conversion obtained using acomparable catalyst that had not undergone hydrothermal or thermaltreatment but had been calcined at 500° C. for two hours. The SCR-activeiron-containing molecular sieves, preferably a zeolite or a SAPO, thathad received hydrothermal treatment or thermal treatment, can alsoincrease NO_(x) conversion over the temperature range of 250-300° C.that is at least about two times, preferably at least about three timesthe conversion obtained using a comparable catalyst that had notundergone hydrothermal or thermal treatment but had been calcined at500° C. for two hours. The SCR-active iron-containing molecular sieves,preferably a zeolite or a SAPO, that had received hydrothermal treatmentor thermal treatment, can also increase NO_(x) conversion over thetemperature range of 200-250° C. that is at least about two times,preferably at least about three times the conversion obtained using acomparable catalyst that had not undergone hydrothermal or thermaltreatment but had been calcined at 500° C. for two hours.

The SCR-active iron-containing molecular sieves, preferably a zeolite ora SAPO, that had received hydrothermal treatment or thermal treatment,can convert about 10, 50 and 90% NOx at about 170, 240 and 280° C. Thetemperatures needed for these conversions are at least 40° C. lower thanthe temperature needed for the same conversion using a comparablecatalyst that had not undergone hydrothermal or thermal treatment (about220, 300 and 350° C.). The SCR-active iron-containing molecular sieves,preferably a zeolite or a SAPO, that had received hydrothermal treatmentor thermal treatment, also provide maximum NO_(x) conversion at about310° C., which is at least 60° C. lower than the temperature needed tomaximum conversion in a comparable catalyst that had not undergonehydrothermal or thermal treatment (about 375° C.).

SCR-active iron-containing molecular sieves, preferably a zeolite or aSAPO, according to the invention, can exhibit a selective catalyticreduction of NO_(x) with NH₃ or urea at 200° C. with an ammonia fillbetween about 0.2 and 0.6 g/L that is at least 10%, preferably at least15% and more preferably at least 20%, greater than a comparableiron-containing molecular sieve that was not hydrothermally or thermallytreated but was calcined at 750° C. for five hours. Preferably, theabove increases in NOx conversion occur with ammonia fills between about0.2 and 1.0 g/L.

The SCR-active iron-containing molecular sieve, preferably a zeolite ora SAPO, according to the invention, preferably has the iron present inthe iron-containing molecular sieve having a higher ratio of Fe²⁺ toFe³⁺ than if the iron-containing molecular sieve were calcined at about500° C. for about two hours without performing the hydrothermal orthermal treatment. Preferably, the iron-containing zeolite or SAPOhaving the higher ratio of Fe²⁺ to Fe³⁺ is a ferrierite, a ZSM-5 orSAPO-34. The iron-containing molecular sieve, preferably a zeolite or aSAPO, can also only contain Fe³⁺, as evidenced by Mossbauerspectroscopy.

An SCR-active iron-containing ferrierite has a Mossbauer spectrumcomprising three doublets having isomer shifts (CS) and quadrupolesplitting (QS) of: (a) CS=0.35 mm/s and QS=1.09 mm/s; (b) CS=0.44 mm/sand QS=2.18 mm/s, and (c) CS=1.2 mm/s and QS=2.05 mm/s, where the valuesfor CS and QS are ±0.02 mm/s.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the invention will become moreapparent after reading the following detailed description of examples ofthe invention, given with reference to the accompanying drawings.

FIG. 1 is a graph illustrating the NO_(x) conversion using anSCR-active, iron-containing zeolite that had been prepared using ahydrothermal treatment according to the invention, in comparison to aconventionally-prepared iron-containing zeolite that did not receive ahydrothermal treatment, but was calcined at about 500° C. for about 2hours.

FIG. 2 is a plot obtained by applying diffuse-reflectance UV-Visspectroscopy to powder samples of the iron-containing zeolitesrepresented in FIG. 1.

FIGS. 3a and 3b are plots obtained by performing Mossbauer spectroscopyto powder samples of the iron-containing zeolites represented in FIG. 1.

FIG. 4 is a graph comparing the NOx conversion as a function of NH₃fill, for each of a series of iron-containing zeolite samples preparedunder respectively different processing conditions.

FIG. 5 schematically depicts an example of a rotary calcining oven thatcan be used to perform the hydrothermal treatment or thermal treatmentfollowed by exposure to oxygen.

EXAMPLE 1

3 wt % iron was added to a commercially available ferrierite zeolite byspray drying the ferrierite zeolite with ammonium Fe (III) oxalate insolution so as to give the desired Fe loading. One portion of theresulting iron-containing ferrierite was dried at 105° C. overnight andwas activated under a flow of 10% steam in nitrogen at 800° C. for 1hour. This material was not subjected to calcination. Another portion ofthe iron-containing ferrierite was calcined at 500° C. in air for 2hours to use as reference.

In the examples that follow, powder samples of the catalysts wereobtained by pelletizing the original samples, crushing the pellets andthen passing the powder obtained through a 255 and 350 micron sieves toobtain a powder having particle size between 255 and 350 microns. Thepowder samples were loaded into a Synthetic Catalyst Activity Test(SCAT) reactor and tested using the following synthetic diesel exhaustgas mixture (at inlet) including nitrogenous reductant: 500 ppm NO, 550ppm NH₃, 12% 02, 4.5% H₂O, 4.5% CO₂, 200 ppm CO, balance N₂ at a spacevelocity of 330 liters per gram of powder catalyst per hour. The sampleswere heated ramp-wise from 150 to 550° C. at 5° C./min and thecomposition of the off-gases detected and the activity of the samples topromote NOx reduction was thereby derived.

As shown in FIG. 1, iron ferrierite in the form of a powder catalystthat was subjected to the hydrothermal treatment under a flow of 10%steam in nitrogen at 800° C. for 1 hour, displays markedly superior lowtemperature (from about 175-300° C.) conversion of NO_(x), as comparedto the iron ferrierite that was not so treated, and instead wassubjected to conventional calcination at 500° C. in air for 2 hours. Thecatalyst subjected to the hydrothermal treatment produced about threetimes the conversion of NOx from compared to the conventionally treatedcatalyst from about 175° C. to about 250° C., (see lines (a)-(c) whichshow the amounts of NOx conversion at 175, 200 and 250° C.,respectively). The amount of conversion using the catalyst subjected tohydrothermal treatment at 175, 200 and 250° C. was about 15, 25 and 60%,respectively, while the amount of conversion using the conventionallytreated catalyst was about 5, 8 and 18%, respectively. At 300° C., theamount of conversion using the catalyst subjected to hydrothermaltreatment was about twice that from the conventionally treated catalyst(>95% versus 50%)

FIG. 1 also shows that catalysts subjected to hydrothermal treatmentcould convert comparable amounts of NOx at significantly lowertemperatures than conventionally treated catalyst. Temperatures neededfor 10, 50 and 90% NOx conversion in catalysts subjected to hydrothermaltreatment were about 170, 240 and 280° C. but were about 220, 300 and350° C. for conventionally treated catalyst. The lowest temperature formaximum NOx conversion was about 310° C. for catalyst subjected tohydrothermal treatment but was about 375° C. for conventionally treatedcatalyst.

These results demonstrate that catalyst subjected to hydrothermaltreatment can produce significantly higher NOx conversion compared to acomparable conventionally treated catalyst. Catalysts subjected tohydrothermal treatment convert similar amounts of NOx at much lowertemperatures compared to a comparable conventionally treated catalyst.

The same powder samples were analyzed using diffuse-reflectance UV-Visspectroscopy in a Perkin-Elmer Lambda 650S spectrometer equipped with anintegrating sphere using BaSO4 as a reference. The samples were placedand packed in a holder. The scan interval was set to 1 nm from 190 to850 nm, the response time was 0.48 sec and a 10% beam attenuator wasused in the reference beam. The data was converted to Kubelka-Munk andnormalised to 5 to the maximum ordinate. The resulting plots are shownin FIG. 2, in which the curves are normalized to the maximum ordinate.These plots indicate that the activation of the iron ferrierite catalystin steam and N₂, leads to significant redispersion of larger Fe speciesinto more active Fe sites, as is shown by the reduction in reflectancefrom 300-400 nm, where oligonuclear species are measured and in theregion above 400 nm, where small clusters of iron oxide and larger Fe₂O₃species are measured.

Powder samples were also analyzed using Mossbauer spectroscopy. ⁵⁷FeMossbauer spectroscopy was performed at room temperature using a Wisselconstant acceleration spectrometer in transmission mode using a 57Cosource in a rhodium matrix. The spectrometer was calibrated relative toα-Fe. The samples were dried and placed in a holder that was gluedclosed. Mossbauer data were collected over a velocity range of +/−6 mms⁻¹ and for different periods of time depending on the sample. Acalibration run was performed on an α-Fe foil over the same velocityrange. All isomer shift values were reported relative to α-Fe andspectra were analysed using the Lorentzian line-shapes facility ofRECOIL software [Lagarec K and Rancourt D G, Recoil: Mossbauer spectralanalysis software for Windows. http://www.isapps.ca/recoil/]. Theresulting spectra is shown in FIGS. 3a and 3b . FIG. 3a is a spectrum ofthe conventionally calcined catalyst (without hydrothermal treatment).The spectrum has two doublets, both having parameters indicative ofFe(III) in an octahedral environment as shown by an isomer shift(CS)=0.35 mm/s and quadrupole splitting (QS)=0.65 mm/s and CS=0.34 mm/sand QS=0.99 mm/s, respectively. The spectra has two doublets ofapproximately equal intensity centered at about −0.05 and about 0.69 mms⁻¹. FIG. 3b is a spectrum of the hydrothermally activated catalyst.This spectrum displays an additional doublet (CS=1.2 mm/s and QS=2.8mm/s) not found in the spectrum from the sample produced withouthydrothermal treatment (FIG. 3a ). The parameters of the additionaldoublet are indicative of Fe(II) in a possibly octahedral environment asindicated by the values CS=1.2 mm/s and QS=2.8 mm/s. Typical values forisomer shifts for Fe(II) are between 0.7 and 1.4 mm/s and for Fe(III)are between 0.1 and 0.6 mm/s. (Edyta Tabor, Karel Zaveta, Naveen K.Sathu, Zdenka Tvaruzkova, Zdenek Sobalík; Catalysis Today 169 (2011)16-23) One of ordinary skill in the art would recognize that both thelocation of the peaks and the intensity of the peaks can vary dependingon numerous factors, including, but not limited to, the age of thesource, the length of time of data acquisition, the presence of water inthe sample, Fe loadings, as well as the type of molecular sieve used.

The spectrum of the iron ferrierite activated according to the inventionshows that some Fe³⁺ species that are present in the conventionallycalcined iron ferrierite convert to Fe²⁺ during activation at hightemperature in H₂O/N₂.

EXAMPLE 2

An iron ferrierite was made as described above by combining 3 wt % ironwith a commercially available ferrierite zeolite by spray drying theferrierite zeolite with ammonium Fe (II) sulphate in solution so as togive the desired Fe loading. A series of powder samples were thenprepared by treating the iron ferrierite at the temperatures andatmospheric conditions as shown in Table 1. Samples were prepared fordetermining their catalytic activity by coating the powder onto ceramiccores.

TABLE 1 Conditions for Preparing Modified Iron Zeolite Temperature (°C.) Time Atmosphere Treatment 800 1 h  2% H₂ + N₂ Thermal 800 1 h 10%H₂O + N₂ Hydrothermal 850 1 h 10% H₂O + N₂ Hydrothermal 850 2 h 10%H₂O + N₂ Hydrothermal 750 5 h Air Calcination (Reference)

The reference sample was prepared by calcination at 750° C. because theferrierite zeolite was treated with ammonium Fe (II) sulphate and ahigher temperature was needed to remove the sulfate.

The test conditions for the data shown in FIG. 4 were NO_(x) conversionat 200° C. as a function of NH₃ fill, with 75% NO₂/NOx, space velocity60 k/hr, and alpha ratio 1.5. These test conditions differ from thoseused to produce the data shown in FIG. 1 at least because of adifference in the composition of the gas. The gas used to generate thedata in FIG. 1 contained NO as the only NOx compound, whereas the gasused to generate the data in FIG. 4 contained a mixture of NO and NO₂,where NO₂ accounted for about 75% of the total NOx. One of ordinaryskill in the art would recognize that the rates of conversion using onlyNO and not NO₂ are slower than the rates of conversion using both NO andNO₂, and therefore rates of conversion measured using only NO areworst-case conversion rates.

As shown in FIG. 4, a substantial (10-20%) increase in NOx conversionwas observed with ammonia fill between about 0.2 and 0.6 g/L. The threecatalysts produced using hydrothermal treatment in a mixed atmospherecomprising steam and nitrogen produced the highest differences comparedto the catalyst that did not undergo hydrothermal treatment. NOxconversion using catalysts that underwent hydrothermal treatment attemperatures of about 800 to about 850° C. in a mixed atmosphere ofsteam and nitrogen for about one hour was about 15 to 20% greater thanfrom a comparable catalyst that did not receive a hydrothermal treatmentand had only been treated in air (calcined). The catalyst producing inthe reducing atmosphere (2% H₂ in N₂), without steam treatment, producedconversions comparable to the sample that received hydrothermaltreatment at 850° C. for 1 hour.

FIG. 5 schematically depicts an example of a rotary calcination oven 10that is well-suited to perform the hydrothermal treatment according tothe invention. The oven 10 is generally cylindrical, and mounted forrotation about its axis. The oven 10 is inclined at an angle θ of about1 to about 15°. An iron-containing molecular sieve, preferably a zeoliteor a SAPO, in powder form is introduced at the higher end of oven 10, asindicated by arrow 11. As the oven is rotated, the iron-containingmolecular sieve moves downwardly through the rotating oven in thedirection of arrow 14. A countercurrent flow of the non-oxidizingatmosphere is provided, as indicated by the arrow 12 in FIG. 5. Theactivated iron containing molecular sieve is then removed from the oven10, as indicated by the arrow 15. Steam is added with the countercurrentnon-oxidative or reductive gas when a hydrothermal treatment isperformed.

The heating of the calcination oven is preferably controlled such thatthree heating zones a, b and c are maintained. In zone a, thetemperature increases from 25° C. at the oven inlet to the temperatureat which the hydrothermal or thermal treatment is performed. In zone b,the temperature is maintained at the temperature at which thehydrothermal or thermal treatment is performed. In zone c, thetemperature decreases from the temperature at which the hydrothermal orthermal treatment is performed, to about 25° C. at the oven outlet.

If an atmosphere of for example nitrogen and steam, or nitrogen andhydrogen, is maintained exclusively within the oven, the activated ironcontaining zeolite has a grey-black color as it emerges from the ovenoutlet. Following coating on a substrate and ordinary calcination in airat about 500 to about 600° C., the color of the iron containing zeolitechanges from grey-black to orange-beige.

It has been unexpectedly discovered that if the treatment in the oven 10is modified so as to permit, a small amount of oxygen to enter the ovenafter the hydrothermal treatment, as shown by the arrow 18 in FIG. 5,the performance of the iron containing zeolite thus processed is furtherimproved. The amount of oxygen admitted near the outlet of the oven 10is in the range of about 1 ppm to about 200,000 ppm above the ambientlevel of oxygen in the oven.

When the treatment concludes with a controlled inclusion of oxygenwithin the oven above ambient levels, it was found that the ironcontaining zeolite exiting the oven can have a different color than thematerial before the treatment, and that the conversion efficiency of theiron containing zeolite is better than when oxygen was not introducedinto the oven. This is the case even after the iron-containing zeolitesprocessed in the oxygen-free oven undergo a subsequent calcination, asdescribed above.

It will be understood that the foregoing description and specificexamples shown herein are merely illustrative of the invention and theprinciples thereof, and that modifications and additions may be easilymade by those skilled in the art without departing from the spirit andscope of the invention, which is therefore understood to be limited onlyby the scope of the appended claims.

What is claimed is:
 1. An SCR-active iron-containing molecular sieve,the iron-containing molecular sieve having undergone a hydrothermaltreatment in a steam-containing non-oxidizing atmosphere, or a thermaltreatment in a reducing atmosphere, at a temperature in the range of600-900° C. for a time period from 5 minutes to two hours, and whereinthe iron containing molecular sieve exhibits a selective catalyticreduction of nitrogen oxides with NH₃ or urea at 250° C. that is atleast 50% greater than a comparable iron-containing molecular sieve thatwas calcined at 500° C. for two hours without performing thehydrothermal treatment in the non-oxidizing atmosphere or the thermaltreatment in the reducing atmosphere, wherein the molecular sieve is azeolite or a silicoaluminophosphate (SAPO); wherein the molecular sieveis a BEA, FER, CHA, AFX, AEI, SFW, SAPO-34, SAPO-56, SAPO-18, SAPO SAV,or SAPO STA-7.
 2. The SCR-active iron-containing molecular sieveaccording to claim 1, wherein the molecular sieve is small-pore ormedium pore.
 3. The SCR-active iron-containing molecular sieve accordingto claim 1, wherein the selective catalytic reduction is at least twotimes greater than if the iron-containing molecular sieve were calcinedat 500° C. for two hours without performing the hydrothermal treatmentin the non-oxidizing atmosphere or the thermal treatment in the reducingatmosphere.
 4. The SCR-active iron-containing molecular sieve accordingto claim 1, wherein the selective catalytic reduction is at least threetimes greater than if the iron-containing molecular sieve were calcinedat 500° C. for two hours without performing the hydrothermal treatmentin the non-oxidizing atmosphere or the thermal treatment in the reducingatmosphere.
 5. The SCR-active iron-containing molecular sieve accordingto claim 1, wherein the iron containing molecular sieve exhibits aselective catalytic reduction of nitrogen oxides with NH₃ or urea at200° C. that is at least 50% greater than if the iron-containingmolecular sieve were calcined at 500° C. for two hours withoutperforming the hydrothermal treatment in the non-oxidizing atmosphere orthe thermal treatment in the reducing atmosphere.
 6. The SCR-activeiron-containing molecular sieve according to claim 1, wherein the ironcontaining molecular sieve exhibits a selective catalytic reduction ofnitrogen oxides with NH₃ or urea at 200° C. that is at least two timesgreater than if the iron-containing molecular sieve were calcined at500° C. for two hours without performing the hydrothermal treatment inthe non-oxidizing atmosphere or the thermal treatment in the reducingatmosphere.
 7. The SCR-active iron-containing molecular sieve accordingto claim 1, wherein the iron present in the molecular sieve has a higherratio of Fe²⁺ to Fe³⁺ than if the iron-containing molecular sieve werecalcined at 500° C. for two hours without performing the hydrothermaltreatment in the non-oxidizing atmosphere or the thermal treatment inthe reducing atmosphere.
 8. The SCR-active iron-containing molecularsieve according to claim 1, wherein the molecular sieve is a zeolite andthe iron present in the zeolite has a higher ratio of Fe²⁺ to Fe³⁺ thanif the iron-containing zeolite were calcined at 500° C. for two hourswithout performing the hydrothermal treatment in the non-oxidizingatmosphere or the thermal treatment in the reducing atmosphere.
 9. TheSCR-active iron-containing molecular sieve according to claim 1, whereinthe molecular sieve is a ferrierite and the iron present in theferrierite has a higher ratio of Fe²⁺ to Fe³⁺ than if theiron-containing zeolite were calcined at 500° C. for two hours withoutperforming the hydrothermal treatment in the non-oxidizing atmosphere orthe thermal treatment in the reducing atmosphere.
 10. An SCR-activeiron-containing molecular sieve, the iron-containing molecular sievehaving undergone a hydrothermal treatment in a steam-containingnon-oxidizing atmosphere, or a thermal treatment in a reducingatmosphere, at a temperature in the range of 600-900° C. for a timeperiod from 5 minutes to two hours, and wherein the iron containingmolecular sieve exhibits a selective catalytic reduction of nitrogenoxides with NH₃ or urea at 250° C. that is at least 50% greater than acomparable iron-containing molecular sieve that was calcined at 500° C.for two hours without performing the hydrothermal treatment in thenon-oxidizing atmosphere or the thermal treatment in the reducingatmosphere, wherein the molecular sieve is a silicoaluminophosphate(SAPO).